Printer with two printing units

ABSTRACT

A printing device, in particular a printer or a copier, which includes a first electrographic printing unit for printing an image pattern on a sheet-type material in a first transfer printing transport path as well as a second electrographic printing unit for printing an image pattern on a sheet-type material in a second transfer printing transport path, and further including an input section via which the sheet-type material can be supplied individually one after the other to both the first and second electrographic printing units, and further having an output section via which the printed sheet-type material from either the first or second electrographic printing units is ejected individually one after the other.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a device or, respectively, a system, in particular a printer or copier, with two electrographic printing units of substantially the same type, wherein each of the printing units prints an image pattern on a sheet-type material. The device includes an input section via which the sheet-type material can be supplied individually in successive fashion and an output section via which the printed sheet-type material is ejected individually in successive fashion.

2. Description of the Prior Art

Electrophotographic printers of this type are known, for example, from DE 34 16 252 A1, EP 0 104 022 A2, DE-PS 1,280,605, DE 34 07 847 A1 or WO 91/13386. With printers of this type, high printing performance can be achieved, even in duplex operation, as long as both printing units are working as they should. However, the devices specified in the cited publications have the disadvantage that, in the case of failure of one of the printing units, a drastic reduction in the performance of the duplex operation results. This is because a sheet that is to be printed on both sides in duplex operation must then be accelerated repeatedly in the opposite direction in order to enable it to be printed on both sides by the one remaining printing unit.

From U.S. Pat. No. 5,150,167, a printing unit is known that contains two printing units. One of the printing units has a priority status. As such, sheets that have been printed in the lower-order printing unit additionally can be printed in the priority printing unit. However, if the priority printing unit fails, double printing and, in particular, duplex printing is not possible.

From U.S. Pat. No. 5,208,640, a further print apparatus is known that contains several printing units. The printing units are connected with one another via a ring system, wherein a common turning apparatus with a shunt for all printing units is used. If the turning apparatus fails, e.g. due to a switching error in the shunt, duplex operation is no longer possible.

An object of the present invention, therefore, is to provide a device, in particular a printer or a copier, that has a high print performance or, respectively, copier performance, in particular in duplex operation.

SUMMARY OF THE INVENTION

This object is solved for the device, or system, named above in that a first printing unit and a substantially identical second printing unit are provided in the device, to which sheet-type material can be supplied via the common input section. The sheet-type material printed by the second printing unit is ejected via a common output section. The two transfer printing transport paths of the two printing units are respectively connected via connecting paths to form two ring systems. A shunt is respectively provided at each of the two rings by means of which the sheet-type material can be turned independent of the respective other shunt in the allocated ring.

In the device of the present invention, two printing units of substantially similar construction are used. The components for these printing units, e.g. the electronic control unit, the developer units, the toner supply and carry-off means, etc., can be retained almost unchanged. Each printing unit, therefore has in itself a very high operational reliability. Both printing units use a common input section via which the sheet-type material is supplied. Likewise, both printing units use in common an output section for carrying away the printed sheet-type material. The new device is thereby constructed very compactly and can be manufactured at a low expense. The print performance is increased significantly due to the two printing units. Also, by means of the inventive ring system, a sheet-type material printed by one of the two printing units can again be supplied to the same printing unit in the same direction as in the first print process.

According to an embodiment of the present invention, a first transfer printing transport path is provided for the first printing unit and a separate second transfer printing transport path is provided for the second printing unit. The transfer printing for both printing units takes place with the same speed. Since each printing unit has a separate transfer printing transport path, sheet-type material can be printed with one printing unit even upon failure of the other.

In addition, it is particularly advantageous if the two rings are connected via two three-way shunts. A multiplicity of transport paths arises by this arrangement.

An embodiment of the present invention provides that the input section contains a shunt that supplies sheet-type material either to the first transfer transport path or to the second transfer printing transport path. In this embodiment, the simplex printing operating mode is performed with one color; i.e., supplied sheet-type material (e.g., individual sheets of paper) are printed on one side by the first printing unit or by the second printing unit. In a further development, the shunt supplies sheet-type material alternately to the first transfer printing transport path and to the second transfer printing transport path. Since each printing unit prints the sheet-type material with the same transfer printing speed, and two printing units are arranged in parallel, the print performance in the device is doubled. Accordingly, individual sheets can be supplied and carried away with twice the print speed.

Another embodiment provides that the first transfer printing transport path and the second transfer printing transport path are connected by a connecting channel through which sheet-type material can be conveyed in one or in both transport directions. By means of these measures, printed material can be supplied from the first printing unit to the second printing unit, and from the second printing unit to the first printing unit, in order to be printed. The connecting channel also creates a feedback that connects the two printing units with one another, thus enabling many print processes.

Another embodiment provides that the sheet-type material is turned during its transport from the first transfer printing transport path to the second transfer printing transport path.

In this way, each printing unit can print both the front side and the back side of an individual sheet. If developer stations with different colors are used for the two printing units, then two image patterns with two different colors can be printed on each side of the individual sheet; i.e., what is known as two-color duplex operation, or duplex color spot operation, can be performed.

According to another embodiment of the present invention, a printer system is specify in which two devices of the same type are connected by an interchange apparatus that supplies the sheet-type material printed by the first device to the second device of the same type. If each of these two devices contains differently colored printing units, then individual sheets can be printed on both sides with four colors. of course as well other variants are conceivable, e.g., one device prints two colors on one side and the other device prints two colors on the other side.

Additional features and advantages of the present invention are described in, and will be apparent from, the Detailed Description of the Preferred Embodiments and the Drawing.

DESCRIPTION OF THE DRAWING

FIG. 1 schematically shows the construction of a high-performance printer in accordance with the present invention;

FIG. 2 schematically shows the operating mode simplex printing with the lower printing unit;

FIG. 3 schematically shows the operating mode simplex printing with the upper printing unit;

FIG. 4 shows the operating mode alternating simplex printing;

FIG. 5 shows the operating mode duplex printing;

FIG. 6 shows the operating mode two-color simplex printing;

FIG. 7 schematically shows the operating mode two-color duplex printing;

FIG. 8 shows an arrangement with two high-performance printers that are connected with one another by an interchange apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a high-performance printer 10 that serves for the rapid printing of individual sheets of paper. The high-performance printer 10 contains a first, lower printing unit D1, as well as a second, upper printing unit D2. Both printing units D1, D2 operate according to the known electrographic method with the same transfer printing speed. Fixing means, indicated schematically in FIG. 1 by two roller pairs 12, 14, are connected downstream from the printing units D1, D2. A paper input 16, containing several supply containers 18 to 24 with individual sheets as well as an external paper input channel 26 via which individual sheets can be supplied from the outside, is connected to the high-performance printer 10. Individual sheets are supplied to an input section 28 via a transport channel. At the output side, a paper output 30 containing several output containers 32 to 36 is connected to the high-performance printer 10. In addition, two output channels 38, 40 are provided via which individual sheets can be outputted to stations that carry out further processing. The high-performance printer 10 ejects the printed individual sheets via the output section 42.

In the interior of the high-performance printer 10, transport paths are arranged for the transport of the individual sheets, by means of which various operating modes of the high-performance printer are enabled. Transfer printing transport paths 44, 46 are respectively allocated to the printing units D1, D2, which are respectively set by means of drives in such a way that the supplied individual sheets at the printing units D1, D2 have their transfer printing speed. Both transfer printing transport paths 44, 46 are connected with one another via a connecting channel 48. The transport path about the first printing unit D1 is supplemented to form a ring R1 by a supply channel 50 via which individual sheets also can be supplied to the second transfer printing transport path 46 from the input section 28. The transport path for the second printing unit D2 is in a similar way supplemented by a carry-off channel 52 to form a ring R2, via which individual sheets printed by the printing unit D1 can be supplied to the output section 42.

A first shunt W1 is arranged between the input section 28, the first transfer printing transport path 44 and the supply channel 50. The first shunt W1 makes it possible for individual sheets to be supplied optionally to the first transfer printing transport path 44 or to the supply channel 50 from the input section 28. A further variant is that individual sheets transported on the supply channel 50 in the direction of the shunt W1 can be supplied to the first transfer printing transport path 44.

In addition, a second shunt W2 and a third shunt W3 are arranged at the ends of the connecting channel 48 wherein they respectively connect the adjoining transport paths 44, 48, 52 or, respectively, 46, 48, 50. A fourth shunt W4 is located in the vicinity of the output section 42 wherein it connects the adjoining transport paths. The paper output 30 contains a fifth shunt W5 that operates as a turning means. In addition, a modulation means 54 is provided, to which rejected individual sheets are supplied via a shunt W6.

By means of the arrangement specified in FIG. 1, various operating modes of the high-performance printer 10 can be enabled. In the following FIGS. 2 to 7, the various operating modes are shown schematically. The respective conveying of the individual sheets is illustrated on the basis of arrows.

FIG. 2 schematically shows simplex printing with only one printing unit. In this simplex printing, only one side of an individual sheet is printed. The individual sheet moves via the input section 28 and the correspondingly switched shunt W1 along the arrow P1 to the first transfer printing transport path 44. It is printed at the printing unit D1. Subsequently, the individual sheet is ejected (arrow P3) into the paper output 30 along the carry-off channel 52 (arrow P2) via the output section 42.

FIG. 3 shows simplex printing with the upper, second printing unit D2. The transport of the individual sheet takes place via the supply channel 50 (arrow P4), the second transfer printing transport path (arrow P5) to the paper output 30 (arrow P6).

In alternating simplex printing with increased print performance, individual sheets are supplied to the printing units D1, D2 via the input unit 28 with at least twice the transfer printing speed. FIG. 4 schematically shows the transport of the individual sheets. The shunt W1 alternately guides individual sheets to the supply channel 50 or, respectively, to the first transfer printing transport path 44 (arrows P7, P8). The individual sheets are first braked to transfer printing speed on their transport up to the printing units D1, D2, are respectively printed there on the front side, and are subsequently further conveyed to the shunt W4. During this further conveying according to the arrows P9, P10, the individual sheets are accelerated to at least double the transfer printing speed so that at the common output section they are ejected via the shunt W4 with a spacing from one another. Also, in the paper output 30, they can be further transported one after the other according to the arrow P11 with at least twice the transfer printing speed.

In the operating mode called “alternating simplex printing,” it is thus provided according to the present invention that in the paper input 16 the individual sheets at the input section 28 are supplied to the printing units D1, D2 with at least twice the transfer printing speed. In the paper output 30 as well, the individual sheets are likewise further conveyed and deposited with at least double the speed. By means of these measures, the individual sheets arrive at the common input section 28 and at the common output section 42 without the possibility of the occurrence of a collision of individual sheets and, accordingly, a paper jam. Preferably, the transport paths for the individual sheets supplied to the first printing unit D1 and the individual sheets supplied to the second printing unit D2 are of symmetrical construction, or are at least equal in length so that on both transport paths the individual sheets can be braked and accelerated with the same speed profile. In this way, it is possible to construct the drives and apparatuses required for the transport in the same way. In addition, it is possible to use control units of the same type.

FIG. 5 schematically shows duplex print operation in which the individual sheets are printed on both sides. The individual sheets supplied to the input section 28 are supplied to the first transfer printing transport path 44 by means of the first shunt W1 (arrow P13). After printing by the first printing unit D1, the respective individual sheet is conveyed along a turning over path according to the arrow P14 via the shunt W2. This turning over path is a part of the carrying-off channel 52. Subsequently, the conveying direction is reversed according to arrow P15, and the shunt W2 then guides the individual sheet into the connecting channel 48 according to the arrow P16. The individual sheet is then diverted to the second transfer printing transport path 46 in the direction of the arrow P17 by the shunt W3. The not-yet-printed back of the individual sheet is thus supplied to the printing unit D2 for printing. Subsequently, the individual sheets are supplied to the shunt W4 according to the arrow P18, and are transported into the paper output 30 along the arrow P19. Since in this state the individual sheet is transported with its back side up, it still has to be turned over before being deposited into the compartments 32 to 36. The shunt W5 serves for this purpose. First, the individual sheet is guided by the shunt W5 in the direction of the arrow P20 for a predetermined turning over path. The transport direction according to the arrow P21 is then reversed, and the shunt W5 conveys the individual sheet in the direction of the arrow P22, whereupon it is deposited in side-correct fashion in the deposit compartments 32 to 36.

As can be seen, the shunt W2 operates as a turning over apparatus in order to supply the back of the individual sheet to the printing unit D2. Alternatively, the shunt W3 also can be used for turning over the sheet. The individual sheet leaving the printing unit D1 is then guided via the shunt W2, the connecting channel 48 of the shunt W3, and then, for a short turning over path, along the supply channel 50 in the direction of the shunt W1. Subsequently, the transport direction is reversed and the shunt W3 guides the individual sheet in the direction of the printing unit D2 with its back side facing up.

FIG. 6 schematically shows a further mode of operation, two-color simplex printing, in which the front of an individual sheet is printed with two image patterns of different colors. The two printing units D1, D2 print image patterns of different colors. In the named operating mode, two-color simplex printing, the individual sheet is supplied to the printing unit D1 via the shunt W1 (arrow P25). Subsequently, the individual sheet is supplied via the shunt W2 to the connecting channel 48, without turning over the sheet, and is then supplied to the printing unit D2 via the shunt W3 (arrows P26, P27). The printing unit D2 prints the front with a color different from the color of the printing unit D1. Subsequently, the individual sheet is ejected to the paper output 30 via the shunt W4 (arrow P28).

FIG. 7 schematically shows the transport path of an individual sheet in the operating mode two-color duplex printing, in which the front and the back of an individual sheet are printed with image patterns of different colors. A precondition of this is that the printing units D1 and D2 print multicolored images. For the two-color printing of the front, the procedure is the same as for the operating mode two-color simplex printing as described with reference to FIG. 6. The arrows P25, P26, P27 and P28 illustrate the transport path. Subsequently, the individual sheet is again supplied to the printing unit D1. The arrows P29 to P36 illustrate the transport path of the individual sheet for the printing of the back. So that this back side is supplied to the printing unit D1, the individual sheet must be turned over on the transport path between the printing unit D2 and the printing unit D1. This turning over can, for example, take place at the shunt W4, the shunt W2 or the shunt W3. In a preferred embodiment of the present invention, the turning over takes place using the shunt W4, i.e., the individual sheet is first transported for a short turning over path in the direction of the shunt W5 whereat the direction of transport is then reversed and the individual sheet is conveyed further in the direction of the shunt W2. After the transport into the paper output 30 according to the arrow P36, a further turning takes place by means of the shunt W5, and the side-correct depositing of the individual sheet, printed on both sides with, respectively, two color images, subsequently takes place.

An alternative transport of the individual sheet through the high-performance printer 10 for the realization of the operating mode two-color duplex printing can take place in the following manner. First, the individual sheet is supplied to the printing unit D1 from the input section 28 via the shunt W1, its front is printed, and it is subsequently guided briefly in the direction of the shunt W1 for turning, via the shunts W2 and W3. After passing the shunt W3, the direction of transport is changed in the direction of the printing unit D2, and the individual sheet is conveyed on the transfer printing transport path 46. The shunt W3 thus serves as a turning over station. The back of the individual sheet is accordingly printed at the printing unit D2. Subsequently, the individual sheet is again supplied to the first printing unit D1 via the shunts W4, W2, W3 and W1, in order now to print the back. The individual sheet must be turned over for this purpose. This takes place at the shunt W4, where it is conveyed briefly in the direction of the shunt W5, the direction of transport is reversed, and it is transported in the direction of the shunt W2 in the turned state. After the printing of the back of the individual sheet in the printing unit D1, the individual sheet is supplied to the printing unit D2 via the shunts W2 and W3, wherein it is turned. Now the front side is printed by the printing unit D2. Subsequently, the individual sheet is guided to the deposit compartments 32 to 36 via the shunt W4. Since it now moves in the correct position, i.e. with the upper side up, into the depository 30, it does not need to be turned over again by the shunt W5.

FIG. 8 shows the arrangement of two high-performance printers 10 and 10′ to form a printing system 55. The two high-performance printers 10 and 10′ are of identical construction so that the individual components do not need to be explained again. An interchange apparatus 56 is arranged between the ejection channel 38 of the first high-performance printer 10 and the external paper input channel 26 of the second high-performance printer 10′, which apparatus conveys individual sheets from the high-performance printer 10 to the high-performance printer 10′. Each high-performance printer 10, 10′ can operate in the already-described operating modes simplex printing, alternative simplex printing, two-color simplex printing, one-color simplex printing and two-color duplex printing. By combining the various operating modes, wherein a first operating mode is set in the high-performance printer 10′ and an agreeing or other operating mode is set in the high-performance printer 10′, new variants of operating modes are enabled. For example, both high-performance printers can operate in two-color simplex operation, wherein the first printer prints the front with two differently colored image patterns and the high-performance printer 10 prints the back with two differently colored image patterns. If respectively different colors are selected for a total of four different printing units, the printing system 55 can accomplish a four-color duplex printing; i.e., the front and the back respectively can be printed with four differently colored image patterns.

The printing system 55 further can be expanded in that at least one additional high-performance printer of the type of the high-performance printer 10 is connected to the high-performance printer 10′. In this way, with the use of all the technological possibilities of the high-performance printer, an n-color duplex printing is enabled in which an individual sheet is printed on the front and on the back with n different colors, where n is an arbitrary whole number.

Although the present invention has been described with reference to specific embodiments, those of skill in the art will recognize that changes may be made thereto without departing from the spirit and scope of the invention as set forth in the hereafter appended claims. 

What is claimed is:
 1. A printing device, comprising: a first electrographic printing unit for printing an image pattern on a sheet-type material in a first transfer printing transport path; a second electrographic printing unit for printing an image pattern on the sheet-type material in a second transfer printing transport path; an input section via which the sheet-type material can be supplied individually one after the other to both the first and second electrographic printing units; an output section via which the printed sheet-type material from both the first and second electrographic printing units is ejected individually one after the other; and first and second rings formed from the respective connection of the first and second transfer printing transport paths via connecting paths, wherein the first ring includes a supply channel via which the sheet-type material can be supplied to the second transfer printing transport path from the input section and the second ring includes a carry-off channel via which the sheet-type material printed by the first printing unit can be supplied to the output section, wherein the sheet-type material printed by one of the first and second printing units in a first print process can again be supplied to the same printing unit such that the sheet-type material runs through the same printing unit in the same direction as in the first print process, and wherein on the first ring a first shunt is provided by means of which the sheet-type material in the first ring can be turned over, and on the second ring a second shunt is provided by means of which the sheet-type material in the second ring can be turned over.
 2. A printing device as claimed in claim 1, wherein the two rings are connected with one another via two three-way shunts such that a multiplicity of different transport paths arises for the sheet-type material.
 3. A printing device as claimed in claim 2, further comprising: an input shunt in the input section that supplies the sheet-type material to at least one of the first transfer printing transport path and the second transfer printing transport path.
 4. A printing device as claimed in claim 3, wherein the first shunt supplies the sheet-type material alternately to the first transfer printing transport path and to the second transfer printing transport path.
 5. A printing device as claimed in claim 1, further comprising: a connecting channel which connects the first transfer printing transport path to the second transfer printing transport path and through which the sheet-type material can be conveyed in at least one direction of transport.
 6. A printing device as claimed in claim 5, further comprising: a turning apparatus for turning the sheet-type material during the transport of the sheet-type material from the first transfer printing transport path to the second transfer printing transport path, and vice-versa.
 7. A printing device as claimed in claim 6, wherein at least one turning apparatus is arranged at at least one end of the connecting channel.
 8. A printing device as claimed in claim 6, wherein the turning apparatus includes a shunt such that the sheet-type material is first transported past the shunt on a first transport path in one direction of transport into a turning section and, thereafter, the direction of transport is reversed such that the shunt conveys the sheet-type material to the second transport path in the other direction of transport.
 9. A printing device as claimed in claim 5, wherein the sheet-type material is first supplied to the first transfer printing transport path and, without turning, is supplied to the second transfer printing transport path via the connecting channel and is then ejected.
 10. A printing device as claimed in claim 1, wherein the sheet-type material is first supplied to the first transfer printing transport path and then to the second transfer printing transport path, and wherein the sheet-type material printed by the second printing unit is again supplied to the first transfer printing transport path after being turned, the sheet-type material subsequently being supplied to the second transfer printing transport path without turning, and is then ejected.
 11. A printing device as claimed in claim 5, wherein the first transfer printing transport path, the connecting channel and a supply channel form a closed transport path wherein the supply channel can transport the sheet-type material in both directions and can supply the sheet-type material to the second transfer printing transport path.
 12. A printing device as claimed in claim 5, wherein the second transfer printing transport path, the connecting channel and the carry-off channel for the sheet-type material form a closed transport path such that the carry-off channel can convey the sheet-type material in both directions and can connect the first transfer printing transport path with the output section.
 13. A printing device as claimed in claim 1, wherein the first printing unit prints image patterns with a first color and the second printing unit prints image patterns with a second color different from the first color.
 14. A printing device as claimed in claim 1, further comprising: a turning means which turns the sheet-type material and is arranged after the output section in a direction of conveying of the sheet-type material.
 15. A printing device as claimed in claim 1, further comprising: a paper input arranged before the input section in the direction of conveying, the paper input providing the sheet-type material in a plurality of supply reservoirs.
 16. A printing device as claimed in claim 1, further comprising: a paper output arranged after the output section in a direction of conveying, the paper output containing a plurality of supply containers into which the ejected sheet-type material is deposited.
 17. A printing device as claimed in claim 5, further comprising: a first shunt contained in the input section; a second shunt arranged at a connection point between the first transfer printing transport path, the connecting channel and the carry-off channel; a third shunt arranged at a connecting point between the connecting channel, the second transfer printing transport path and the supply channel; and a fourth shunt arranged at a connecting point between the second transfer printing transport path and the carry-off channel.
 18. A printing device as claimed in claim 17, wherein in an operating mode two-color duplex printing with two colors per side, the sheet-type material is supplied to the first printing unit via the input section and the first shunt and is printed there with the first color, the sheet-type material is transported to the second printing unit via the second shunt and the third shunt and is printed there with the second color, the sheet-type material is transported to the fourth shunt and into a turning section such that the direction of transport is reversed and changed to the direction of the second shunt, the sheet-type material is supplied to the first printing unit via the third shunt and the first shunt and is printed on a second side with the first color, the sheet-type material is transported to the second printing unit via the second shunt and the third shunt and is printed on the second side with the second color, and the sheet-type material is ejected via the fourth shunt.
 19. A printing device as claimed in claim 17, wherein in an operating mode two-color duplex printing with two colors per side, a front side of the sheet-type material is supplied to the first printing unit via the input section and the first shunt and is printed there with a first color, the sheet-type material is transported, after turning, to the second printing unit via the second shunt and the third shunt and a backside is printed there with a second color, the back of the sheet-type material is supplied to the first printing unit via the fourth shunt, the second shunt, the third shunt, and the first shunt and is printed with the first color, the sheet-type material is supplied, after turning, to the second printing unit via the second shunt and the third shunt and is printed with the second color, and the sheet-type material is ejected via the fourth shunt.
 20. A printing device as claimed in claim 1, wherein the sheet-type material is supplied to the input section with a speed at least equal to twice a transfer printing speed of the printing units such that the sheet-type material is braked to the transfer printing speed before reaching the respective printing unit.
 21. A printing device as claimed in claim 20, wherein the sheet-type material is accelerated to a speed that is at least equal to twice the speed of the transfer printing speed of the printing units in its transport from the respective printing unit to the output section.
 22. A printing system having two printing devices of the type claimed in claim 1, further comprising: an interchange apparatus that is connected between the output section of the first printing device and the input section of the second printing device.
 23. A printing system as claimed in claim 22, wherein both of the printing devices contain printing units that print multi-colored image patterns.
 24. A printing system as claimed in claim 22, further comprising: a third printing device which is substantially the same as the first and second printing devices and which is connected to the second printing device.
 25. A printer system, comprising: a first electrographic printing unit with which an image pattern can be printed on sheet-type material in a first transfer printing transport path; a second electrographic printing unit with which an image pattern can be printed on the sheet-type material in a second transfer printing transport path; an input section via which the sheet-type material can be supplied individually one after the other to both the first and second printing units; an output section via which the printed sheet-type material is individually ejected one after the other from both the first and second printing units; and a carry-off channel allocated to the second printing unit, said carry-off channel being constructed so that the sheet-like material printed on one side by the second printing unit is supplied again to the second printing unit for printing on a backside, and the sheet-type material that was printed by the first printing unit can be supplied to the output section via the carry-off channel in a manner that circumvents the transfer printing transport path of the second printing unit.
 26. A printer system as claimed in claim 25, wherein in a first operating mode in which material that was printed on one side by the second printing unit is again supplied to the second printing unit for printing on a back side, the sheet-type material in the carry-off channel is transported in a first direction, and in a second operating mode in which the sheet-type material that was printed by the first printing unit is supplied to the output section in a manner that circumvents the transfer printing transport path of the second printing unit, the sheet-type material is transported in a second direction opposite to the first direction.
 27. A printer system as claimed in claim 26, wherein in a second operating mode a simplex printing is carried out with increased speed such that individual sheets to be printed are alternately supplied to the first printing unit and the second printing unit for one-sided printing, are ejected with a spacing via a shunt at the common output section, and are further transported one after the other.
 28. A printer system as claimed in claim 25, wherein the carry-off channel is arranged underneath the second printing unit and positioned substantially parallel to the second transfer printing transport path.
 29. A printer system as claimed in claim 25, further comprising: a connecting channel which connects the first transfer printing transport path to the second transfer printing transport path and through which the sheet-type material can be conveyed in at least one direction of transport.
 30. A printer system as claimed in claim 29, wherein the connecting channel and a supply channel form a first closed transport path, such that the supply channel can transport the sheet-type material in both directions and can supply it to the second transfer printing transport path.
 31. A printer system as claimed in claim 30, wherein the second transfer printing transport path, the connecting channel and the carry-off channel form a second closed transport path for the sheet-type material.
 32. A printer system as claimed in claim 31, further comprising: a plurality of shunts for connecting the transfer printing transport paths with at least one of the connecting channel, the supply channel and the carry-off channel, and wherein the connecting channel is used in common by the first and second closed transport path.
 33. A printing system as claimed in claim 25, wherein the first printing unit prints image patterns with a first color, and the second printing unit prints image patterns with a second color different from the first color.
 34. A printing system as claimed in claim 25, further comprising: a turning over apparatus for supplying an individual sheet, printed on a front side, to one of the two printing units for printing on a backside individual sheet.
 35. A printing system as claimed in claim 25, wherein the carry-off channel is arranged underneath the second printing unit.
 36. A printing system as claimed in claim 27, wherein the sheet-type material at the common output section is ejected in alternating fashion.
 37. A printing system as claimed in claim 25, further comprising: a supply channel allocated to the first printing unit via which channel material that was printed on the front side by the first printing unit is supplied to the first printing unit again for printing on the backside and, via the supply channel, the sheet-type material can be supplied to the transfer printing transport path of the second printing unit from the input section so as to circumvent the transfer printing transport path of the first printing unit.
 38. A printing system as claimed in claim 37, further comprising: a shunt provided at the input section via which, in alternating fashion, the sheet-type material is supplied to the transfer printing transport path of the first printing unit and, via the supply channel, to the transfer printing transport path of the second printing unit.
 39. A printing system as claimed in claim 37, wherein the supply channel relating to the first printing unit and the carry-off channel relating to the second printing unit are arranged in opposite directions.
 40. A printing system as claimed in claim 39, wherein the supply channel is arranged above the first printing unit and the carry-off channel is arranged underneath the second printing unit.
 41. A printer system having a transport system for sheet-type recording media, comprising: a first shunt via which the sheet-type recording material can alternately be supplied to one of a transfer printing transport path of a first printing unit and, via a supply channel, to a transfer printing transport path of a second printing unit; a second shunt via which the sheet-type recording material that was printed by the first printing unit can alternately be supplied via a carry-off channel to one of an output section and a connecting channel; a third shunt provided at the connecting channel via which the sheet-type recording material coming from the connecting channel can alternately be supplied to one of the transfer printing transport path of the second printing unit and the supply channel; and a fourth shunt that supplies both the sheet-type material coming from the transfer printing transport path of the second printing unit and the sheet-type material coming from the carry-off channel to the output section.
 42. A printer system as claimed in claim 41, wherein at least one of the first, second, third and fourth shunts is formed as a turning means.
 43. A printer system as claimed in claim 41, wherein the recording material can be transported in the supply channel in two opposed directions.
 44. A printer system as claimed in claim 41, wherein the recording material can be transported in the carry-off channel in two opposed directions.
 45. A printer system as claimed in claim 41, wherein, via the supply channel and the connecting channel, the recording material that was printed on the front side by the first printing unit can be supplied to the first printing unit for printing on the back side.
 46. A printer system as claimed in claim 41, wherein, via the carry-off channel and the connecting channel, the recording material that was printed on the front side by the second printing unit can be supplied to the second printing unit for printing on the back side.
 47. A printer system as claimed in claim 41, wherein the recording material is printed in a first color by the first printing unit and is printed in a second color by the second printing unit.
 48. A printer system as claimed in claim 41, wherein the supply channel runs substantially parallel to the transfer printing transport path of the first printing unit.
 49. A printer system as claimed in claim 41, wherein the carry-off channel runs substantially parallel to the transfer printing transport path of the second printing unit. 